We report the instrument description and the results
of the laboratory calibration and tests of a mid-infrared
tunable diode spectrometer for in situ trace gas
concentration measurements in the stratosphere operating
on a stratospheric aircraft. The spectrometer is dedicated
to the measurement of the HNO3 amount in the
stratospheric aerosols by means of gas-phase absorption
spectroscopy on molecular roto-vibrational lines in the
mid-infrared, using a tunable diode laser and a multipass
absorption cell. The instrument was specifically designed
for operation aboard of the stratospheric aircraft M55
Geophysica, in the frame of the Airborne Platform
for Earth observation (APE) project. The instrument is
part of a measurement package for the measurement of the
chemical content of Polar Stratospheric Clouds (PSCs) and
other atmospheric aerosols. This system can be also used
as a stand-alone detector of molecular trace gases. Design
criteria include an efficient optical layout, with a very
low sensitivity to the vibration and thermal stresses and
a very small footprint, and a detection scheme based on
the sweep integration technique for fast data acquisition
and high signal-to-noise (S/N) ratio.
We report a new set of testing measurements on ammonia as
the calibration gas with one order of magnitude improvement
with respect to what we previously reported.